The present invention relates to a display apparatus, particularly by a liquid crystal display apparatus including a liquid crystal device for use in light-valves for flat-panel displays, projection displays, printers, etc., and a driving method for the (liquid crystal) display apparatus.
As a type of a nematic liquid crystal display device used heretofore, there has been known an active matrix-type liquid crystal device wherein each pixel is provided with an active element (e.g., a thin film transistor (TFT)).
As a nematic liquid crystal material used for such an active matrix-type liquid crystal device using a TFT, there has been presently widely used a twisted nematic (TN) liquid crystal as disclosed by M. Schadt and W. Helfrich, xe2x80x9cApplied Physics Lettersxe2x80x9d, Vol. 18, No. 4 (Feb. 17, 1971), pp. 127-128.
In recent years, there has been proposed a liquid crystal device of In-Plain Switching mode utilizing an electric field applied in a longitudinal direction of the device, thus improving a viewing angle characteristic being problematic in TN-mode liquid crystal displays. Further, a liquid crystal device of a super twisted nematic (STN) mode without using the active element (TFT etc.) has also be known as a representative example of the nematic liquid crystal display device.
Accordingly, the nematic liquid crystal display device includes various display or drive modes. In any mode however, the resultant nematic liquid crystal display device has encountered a problem of a slow response speed of several ten milliseconds or above.
In order to solve the above-mentioned difficulties of the conventional types of nematic liquid crystal devices, a liquid crystal device using a liquid crystal exhibiting bistability (xe2x80x9cSSFLCxe2x80x9d, Surface Stabilized FLC), has been proposed by Clark and Lagerwall (Japanese Laid-Open Patent Application (JP-A) 56-107216, U.S. Pat. No. 4,367,924). As the liquid crystal exhibiting bistability, a chiral smectic liquid crystal or a ferroelectric liquid crystal (FLC) having chiral smectic C phase (SmC*) is generally used. Such a chiral smectic (ferroelectric) liquid crystal has a very quick response speed because it causes inversion switching of liquid crystal molecules by the action of an applied electric field on spontaneous polarizations of their liquid crystal molecules. In addition, the chiral smectic liquid crystal develops bistable states showing a memory characteristic and further has an excellent viewing angle characteristic. Accordingly, the chiral smectic liquid crystal is considered to be suitable for constituting a display device or a light valve of a high speed, a high resolution and a large area.
In recent years, as another liquid crystal material, an antiferroelectric liquid crystal showing tristability (tristable states) has caught attention. Similarly as in the ferroelectric liquid crystal, the antiferroelectric liquid crystal causes molecular inversion switching due to the action of an applied electric field on its spontaneous polarization, thus providing a very high-speed responsiveness. This type of the liquid crystal material has a molecular alignment (orientation) structure wherein liquid crystal molecules cancel or counterbalance their spontaneous polarizations each other under no electric field application, thus having no spontaneous polarization in the absence of the electric field.
The above-mentioned ferroelectric and antiferroelectric liquid crystal causing inversion switching based on spontaneous polarization are liquid crystal materials assuming smectic phase (chiral smectic liquid crystals). Accordingly, by using these liquid crystal materials capable of solving the problem of the conventional nematic liquid crystal materials in terms of response speed, it has been expected to realize a smectic liquid crystal display device.
As described above, the (anti-)ferroelectric (or chiral smectic) liquid crystal having a spontaneous polarization has been expected to be suitable for use in displays exhibiting a high-speed response performance in the near future.
In the case of the above-mentioned device (cell) using the (anti-)ferroelectric liquid crystal exhibiting bistability or tristability, however, it has been difficult to effect a gradation display in each pixel based on its display principle.
In recent years, in order to allow a mode of controlling various gradation levels, there have been proposed liquid crystal devices using a specific chiral smectic liquid crystal, such as a ferroelectric liquid crystal of a short pitch-type, a polymer-stabilized ferroelectric liquid crystal or an anti-ferroelectric liquid crystal showing no threshold (voltage) value. However, these devices have not been put into practical use sufficiently.
On the other hand, with respect to a liquid crystal display apparatus, it has been clarified by recent studies that it is difficult to attain a sufficient human-sensible high-speed motion picture response characteristic only by simply increasing a response speed of a liquid crystal portion of a conventional liquid crystal device (using a nematic TN or STN) mode)(as described in, e.g., xe2x80x9cShingaku Gihoxe2x80x9d (Technical Report of IEICD), EID 96-4 (1996-06, p. 19).
According to results of these studies, it has been concluded that a scheme wherein a time aperture (opening) rate is decreased to at most 50% by using a shutter or a double-rate display scheme is effective in improving motion picture qualities as a scheme by which a human-sensible high-speed motion picture responsiveness is provided.
However, in the conventional nematic (display) mode, the response speed of a liquid crystal is insufficient, thus failing to be applied to the above motion picture display schemes. Further, in order to realize the high-speed motion picture display as described above by using the conventionally proposed high-speed responsive chiral smectic liquid crystal devices including those using a ferroelectric liquid crystal of a short pitch-type or a polymer-stabilized type and a threshold-less antiferroelectric liquid crystal, any (chiral) smectic mode is accompanied with difficulties, such as complicated driving method and peripheral circuits, thus leading to an increase in production cost. Even when a time aperture rate is completely set to 50% or below, the entire display device (apparatus) is also correspondingly decreased in brightness of 50% or below. As a result, it is clear that the resultant display device causes a lowering in (display) luminance.
In recent years, it has been desired to effect full-color display using a liquid crystal device. As one of methods for effecting full-color display, there has been known a method wherein a liquid crystal device is irradiated with respective color lights (e.g., red light, green light and blue light) in succession to effect switching of liquid crystal molecules under the respective color light irradiations. Even in such a liquid crystal device, however, if the time aperture rate is decreased to at most 50% as described above, the resultant liquid crystal device is similarly accompanied with a (display) luminance lowering problem.
More specifically, FIG. 19 is a block diagram of a conventional liquid crystal apparatus.
Referring to FIG. 19, the liquid crystal apparatus includes a liquid crystal device (panel) 80, a color light source 101 capable of emitting respective color lights (of red (R), green (G) and blue (B)) and a color light source driving unit 102 for driving the color light source 101 based on synchronizing signals.
The liquid crystal device 80 shown in FIG. 19 includes 480 scanning lines supplied with scanning (data) signals X001 to X480, respectively, through a Y-driver 92. These X- and Y-drivers 91 and 92 are driven by applying a drive voltage carrying drive signals. The synchronizing signals supplied to the color light source driving unit are separated from the drive signals.
FIG. 20 is a time chart for illustrating a driving method of the conventional liquid crystal apparatus shown in FIG. 19.
Referring to FIG. 20, when the liquid crystal apparatus is driven, one frame period F0 is divided into three field periods F1, F2 and F3. In this instance, when a frame frequency is set to 60 Hz, one frame period F0 is ca. 16.7 msec. and each of the field period F1, F2 and F2 is ca. 5.5 msec. The liquid crystal device 80 is irradiated successively with the respective color lights (R, G, B) from the color light source 101 in the field periods F1, F2 and F3, respectively (FIGS. 20(a), (b) and (c)). In each of the field periods F1, F2 and F3, with respect to each of scanning lines (S001 to S048), a black and white (monochromatic) image (for R in F1, for G in F2 or for B in F3) is successively displayed in a prescribed display period (RD, GD or BD) as shown in FIG. 20(d). As a result, these resultant (color) images are visually color-mixed to be recognized as a desired full-color image.
According to such a liquid crystal apparatus, it is not necessary to provide the liquid crystal device 80 with a color filter, thus obviating problems due to the formation of the color filter, such as a lowering in production yield, an attenuation (lowering in luminance) of illumination light at the color filter and an increase in quantity of light of a backlight (light source) for preventing the lowering in luminance. On the other hand, however, the image display period (Rd, GD or BD) is half of the corresponding field period (F1, F2 or F3), thus resulting in an about half utilization of the color light source 101. Accordingly, the resultant luminance is lowered in spite of no attenuation of the illumination light by the use of the color filter, so that the color light source 101 is required to provide a higher luminance in order to prevent the lowering in luminance of the liquid crystal device 80.
In the case where such a liquid crystal device 80 uses a ferroelectric liquid crystal (e.g., a liquid crystal assuming chiral smectic C phase), it is necessary to apply a reset pulse (voltage) in combination with a writing pulse. Even when the reset pulse is set to have a negative polarity and the writing pulse is set to have a positive polarity, the resultant writing pulse becomes smaller depending on displaying gradation levels in some cases, thus resulting in DC voltage component applied to the liquid crystal to cause an occurrence of so-called burning or sticking.
In view of the above-mentioned problems, an object of the present invention is to provide a display apparatus, particularly a liquid crystal display apparatus, capable of effecting gradation control with high-speed responsiveness while ensuring a practical brightness to improve motion picture image qualities without using a complicated circuit.
Another object of the present invention is to provide a driving method for the (liquid crystal) display apparatus.
According to the present invention, there is provided a display apparatus, comprising:
a display device including a plurality of pixels, and
control means for effecting a plurality of displaying operations at each pixel, each displaying operation including at least a first operation for displaying a first image at a first luminance and a second operation for displaying a second image substantially identical to the first image at a second luminance, said first and second luminances being non-zero and different from each other.
According to the present invention, there is also provided a liquid crystal display apparatus, comprising:
a liquid crystal device including a layer of liquid crystal, a pair of substrates disposed to sandwich the liquid crystal, and a polarizer disposed on at least one of the substrates, at least one of the substrates being provided with an alignment film for aligning the liquid crystal in contact therewith, the pair of substrates respectively having thereon mutually intersecting electrodes for applying a voltage to the liquid crystal thereby forming a matrix of pixels each at an intersection of the electrodes on the pair of substrate, and
control means for effecting a plurality of displaying operations at each pixel, each displaying operation including at least a first operation for displaying a first image at a first luminance and a second operation for displaying a second image substantially identical to the first image at a second luminance, said first and second luminances being non-zero and different from each other.
According to the present invention, there is further provided a liquid crystal apparatus, comprising:
a liquid crystal device including a layer of liquid crystal, a pair of substrates disposed to sandwich the liquid crystal, and a polarizer disposed on at least one of the substrates, at least one of the substrates being provided with an alignment film for aligning the liquid crystal in contact therewith, the pair of substrates respectively having thereon mutually intersecting electrodes for applying a voltage to the liquid crystal thereby forming a matrix of pixels each at an intersection of the electrodes on the pair of substrate,
a light source provided to one of the substrates for emitting light to be optically modulated by the liquid crystal device, and
control means for effecting a plurality of illuminating operations including at least a first operation for displaying a first image by turning the light source on at a first illuminance and a second operation for displaying a second image substantially identical to the first image by turning the light source on at a second illuminance, said first and second illuminances being non-zero and different from each other.
The present invention provides a liquid crystal apparatus, comprising:
a liquid crystal device including a layer of liquid crystal, a pair of substrates disposed to sandwich the liquid crystal, and a polarizer disposed on at least one of the substrates, at least one of the substrates being provided with an alignment film for aligning the liquid crystal in contact therewith, the pair of substrates respectively having thereon mutually intersecting electrodes for applying a voltage to the liquid crystal thereby forming a matrix of pixels each at an intersection of the electrodes on the pair of substrate, and
voltage application means for applying a voltage to the liquid crystal through the electrodes, wherein
the liquid crystal has an alignment characteristic such that the liquid crystal is aligned to provide an average molecular axis to be placed in a monostable alignment state under no voltage application, is tilted from the monostable alignment state in one direction when supplied with a voltage of a first polarity at a tilting angle which varies depending on magnitude of the supplied voltage, and is tilted from the monostable alignment state in the other direction when supplied with a voltage of a second polarity opposite to the first polarity at a tilting angle, said tilting angles providing maximum tilting angles formed under application of the voltages of the first and second polarities, respectively, different from each other.
The present invention also provides a liquid crystal apparatus, comprising:
a liquid crystal device including a layer of liquid crystal, a pair of substrates disposed to sandwich the liquid crystal, and a polarizer disposed on at least one of the substrates, the pair of substrates respectively having thereon mutually intersecting electrodes for applying a voltage to the liquid crystal thereby forming a matrix of pixels each at an intersection of the electrodes on the pair of substrate, and
a drive circuit for driving the liquid crystal device to effect desired gradational display based on change in emitting light quantity for each pixel, wherein each pixel is supplied with a driving signal from said drive circuit, said driving signal including in a first period a voltage of a first polarity for providing a prescribed light quantity equal to or larger than a light quantity for providing a prescribed gradational image and in a second period a voltage of a second polarity opposite to the first polarity for providing a second light quantity smaller than the prescribed light quantity but larger than zero, thereby to effect desired gradational display through the first and second period.
The present invention further provides a driving method for a display apparatus wherein a plurality of color lights are successively emitted from a color light source and in synchronism with the respective light emissions, switching of the respective lights is effected by a display device to visually color-mixing the respective lights to provide a full-color image, said driving method comprising:
dividing one frame period into a plurality of field periods and further dividing each field period into a plurality of sub-field periods,
changing a color of a light emitted from the color light source for each field period, and
displaying a higher luminance image in at least one sub-field period in each field period and a lower luminance image in at least one another sub-field period in each field period.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.